Roger L. Farrow

Measuring fundamental properties in operating solid oxide electrochemical cells by using in situ X-ray photoelectron spectroscopy

Photoelectron spectroscopic measurements have the potential to provide detailed mechanistic insight by resolving chemical states, electrochemically active regions and local potentials or potential losses in operating solid oxide electrochemical cells (SOCs), such as fuel cells. However, high-vacuum requirements have limited X-ray photoelectron spectroscopy (XPS) analysis of electrochemical cells to ex situ investigations. Using a combination of ambient-pressure XPS and CeO(2-x)/YSZ/Pt single-chamber cells, we carry out in situ spectroscopy to probe oxidation states of all exposed surfaces in operational SOCs at 750 °C in 1 mbar reactant gases H(2) and H(2)O. Kinetic energy shifts of core-level photoelectron spectra provide a direct measure of the local surface potentials and a basis for calculating local overpotentials across exposed interfaces. The mixed ionic/electronic conducting CeO(2-x) electrodes undergo Ce(3+)/Ce(4+) oxidation-reduction changes with applied bias. The simultaneous measurements of local surface Ce oxidation states and electric potentials reveal the active ceria regions during H(2) electro-oxidation and H(2)O electrolysis. The active regions extend ~150 μm from the current collectors and are not limited by the three-phase-boundary interfaces associated with other SOC materials. The persistence of the Ce(3+)/Ce(4+) shifts in the ~150 μm active region suggests that the surface reaction kinetics and lateral electron transport on the thin ceria electrodes are co-limiting processes.

Detection of Trace Molecular Species Using Degenerate Four-Wave Mixing

Spectroscopies that make use of laser light have provided an important tool to modern researchers for the nonintrusive analysis of chemical systems. The strengths and limitations of these spectroscopic techniques often determine the viability of scientific investigations. The unique properties of degenerate four-wave mixing, a nonlinear optical technique, have recently been found to provide powerful capabilities for a wide range of applications.

Calculation of collisionally narrowed coherent anti-Stokes Raman spectroscopy spectra

High-resolution coherent anti-Stokes Raman spectroscopy spectra of the N(2) Q branch at 294 K have been obtained at 1, 5, and 10 atm. Even at 1-atm pressure, disagreements with spectra calculated using the isolated line approximation were observed, indicating the importance of collisional narrowing effects in describing these spectra. A method of using the full G-matrix approach for the calculation of these spectra that is both exact and computationally efficient (requiring only one matrix diagonalization and inversion per spectrum) is discussed. Excellent agreement with experimental data is obtained using this method and a simple exponential gap model for the off-diagonal G-matrix elements.

Comparison between CARS and corrected thermocouple temperature measurements in a diffusion flame.

Coherent anti-Stokes Raman spectroscopy (CARS) has been used to obtain radial temperature profiles in an axisymmetric methane diffusion flame. Temperatures were obtained from analysis of background-free nitrogen Q- and O-branch spectra. The spectra were analyzed with a nonlinear least-squares CARS fitting program and compared to measurements from radiation- and conduction-corrected thermocouples. Excellent agreement was obtained in regions of relatively constant temperature, whereas improved CARS spatial resolution was required to obtain agreement near steep temperature gradients.

Investigation of the dependence of degenerate four-wave mixing line intensities on transition dipole moment

We report an investigation of the dependence on transition dipole moment of isolated line intensities in degenerate four-wave mixing in the A2Σ+ (υ′ = 0) ← X2Π (υ″ = 0) band for NO. For spectrally integrated intensities we found that the dependence was well described by a power law μx, where μ is the one-photon transition dipole moment. As a result of saturation, the exponent x depends on laser intensity. The observations are in reasonable agreement with a simple two-level model [ Opt. Lett.294 ( 1978);Opt. Lett.3, 205 ( 1978)], which predicts limiting dependences of μ8 and μ3 for low and high intensities, respectively. At intermediate laser intensities the model is consistent with an observed rapid decrease in the exponent with increasing laser intensity, followed by a plateau extending to high intensities.

Pump–probe measurements of state‐to‐state rotational energy transfer rates in N2 (v=1)

We report direct measurements of the state‐to‐state rotational energy transfer rates for N2 (υ=1) at 298 K. Stimulated Raman pumping of Q‐branch (υ=1←0) transitions is used to prepare a selected rotational state of N2 in the υ=1 state. After allowing an appropriate time interval for collisions to occur, 2+2 resonance‐enhanced multiphoton ionization is used (through the a 1Πg←X 1Σ+g transition) to detect the relative population of the pumped level and other levels to which rotational energy transfer has occurred. We have performed a series of measurements in which a single even rotational level (Ji=0–14) is excited and the time‐dependent level populations are recorded at three or more delay times. This data set is then globally fit to determine the best set of state‐to‐state rate constants. The fitting procedure does not place any constraints (such as an exponential gap law) on the J or energy dependence of the rates. We compare our measurements and best‐fit rates with results predicted from phenomenologic...

Peak-power limits on fiber amplifiers imposed by self-focusing

We have numerically investigated the behavior of the fundamental mode of a step-index, multimode (MM) fiber as the optical power approaches the self-focusing limit (P(crit)). The analysis includes the effects of gain and bending (applicable to coiled fiber amplifiers). We find power-dependent, stationary solutions that propagate essentially without change at beam powers approaching P(crit) in straight and bent fibers. We show that in a MM fiber amplifier seeded with its fundamental eigenmode at powers <<P(crit), the transverse spatial profile adiabatically evolves through a continuum of stationary solutions as the beam is amplified toward P(crit).

The effects of collisional quenching on degenerate four-wave mixing

We report a theoretical and experimental investigation of the effects of collisional quenching on resonant degenerate four-wave mixing (DFWM). Using single-mode laser radiation, peak signal intensity measurements were performed on an isolated line in the A – X transition of NO. By using appropriate mixtures of N2 and CO2 as buffer gases, we varied the collisional quenching rate over several orders of magnitude while maintaining a fixed total collisional dephasing rate. The mixtures had approximately 100 Torr total pressure and were at room temperature. For I/Isat approximately equal to 0.02, DFWM intensities were found to be less affected by variations in quench rate than were laser-induced fluorescence (LIF) intensities (I and Isat are the pump laser and one-photon saturation intensities, respectively). Moreover, for I/Isat roughly equal to 0.5, DFWM intensities were observed to be nearly independent of quench rate. The results are compared to two theoretical predictions, with good agreement observed. Both theories indicate that the minimum sensitivity of DFWM to quenching occurs near I/Isat≈1.

Gas-phase thermal-grating contributions to four-wave mixing

The nature of optical scattering from laser-induced thermal gratings created in the gas phase is investigated. Thermal gratings are produced with the illumination geometry used to perform degenerate four-wave mixing (DFWM) measurements. Such scattering from thermal gratings can act as a phase-matched interference signal. A solution to the linearized hydrodynamic equations is developed to model the dynamics of the thermal grating. Predictions of this model that uses realistic gas properties are shown to compare favorably with laboratory measurements. The model includes the effects of finite-rate energy deposition, damping by viscosity and thermal conduction, mass diffusion of the excited-state grating, and electrostrictive compression.

Two-dimensional imaging of OH in flames by degenerate four-wave mixing

Degenerate four-wave mixing is used to produce two-dimensional images of OH distributions in atmosphericpressure flames. The phase-conjugated images from single laser pulses exhibit excellent signal-to-noise ratios and illustrate that degenerate four-wave mixing has outstanding potential as a multidimensional diagnostic for combustion environments.